nixos/version: fix case where .git is a symlink

Before c9214c394b and
9d396d2e42 if .git is symlink the version
would gracefully default to no git revision. With those changes an
exception is thrown instead.

This introduces a new function `pathIsGitRepo` that checks if
`commitIdFromGitRepo` fails without error so we don't have to
reimplement this logic again and can fail gracefully.

lib/sources.nix

@@ -105,6 +105,7 @@ rec {
       in type == "directory" || lib.any (ext: lib.hasSuffix ext base) exts;
     in cleanSourceWith { inherit filter; src = path; };
 
+  pathIsGitRepo = path: (builtins.tryEval (commitIdFromGitRepo path)).success;
 
   # Get the commit id of a git repo
   # Example: commitIdFromGitRepo <nixpkgs/.git>